It's nothing else than: if someone wants their posts shared by others, they can invite all HivePoshers who were active in the last contest. Nobody has to do this and nobody should re-pay for this. But if you see @theworldaroundme or anybody else invites you, rush to share in the best possible sub - if you believe the suggested post will work well. If you feel the suggested post is weak, you can ignore the GOSH invite.
You can invite too when you want - maybe never, many tomorrow - up to you. For example, you wrote a post - spent 3 hours, shared it, and the post was deleted immediately - you'll earn 0.2 $HIVE or so for 3 hours of work. You can use the GOSH command to invite everyone. If someone shares your post well and earns 20$ Redditposh rewards, you'll have 20% as an author - 4$. That's good for you. Secondly, you'll help HivePosh as a whole and Hive blockchain - more shares, more views, more visits from Reddit to Hive.
The downside is: someone can generate more views than you as the result of your GOSH invite and you'll miss a medal in the HivePosh contest.
I don't mind this, as it is a way to give others a chance while not limiting how much you can do, and it also helps Hiveposh and Hive in general, and it builds teamwork.